With the need for smaller sizes, lower weights and higher functionality in portable electronic devices such as smart phones, digital cameras and handheld game consoles, the development of high-performance batteries has been actively promoted in recent years, and demand for secondary batteries—which can be repeatedly used by charging—is growing rapidly. Lithium-ion secondary batteries in particular, because of their high energy density and high voltage, and moreover because they have no memory effect during charging and discharging, are the secondary batteries currently being most vigorously developed. Electrical car development is also proceeding apace as part of recent efforts to tackle environment problems, and an even higher level of performance is being demanded of the secondary batteries that serve as the power source in such vehicles.
Lithium-ion secondary cells have a structure in which a container houses a positive electrode and a negative electrode capable of intercalating and deintercalating lithium and a separator interposed between the electrodes, and is filled with an electrolyte solution (in the case of lithium-ion polymer secondary cells, a gel-like or completely solid electrolyte instead of a liquid electrolyte solution).
The positive electrode and negative electrode are generally produced by forming a composition which includes an active material capable of intercalating and deintercalating lithium, an electrically conductive material composed primarily of a carbon material, and a binder resin into a layer on a current collector such as copper foil or aluminum foil. The binder is used for bonding the active material with the conductive material, and moreover for bonding these with the metal foil. Exemplary binders that are commercially available include fluoropolymers which are soluble in N-methylpyrrolidone (NMP), such as polyvinylidene fluoride (PVDF), and aqueous dispersions of olefin polymers.
The conductive material included in the positive and negative electrodes, also referred to as a “conductive additive,” is an important material for increasing the electrical conductivity of the active material layer and smoothly carrying battery charging and discharging, although there are several drawbacks to the presence of a conductive additive in an electrode. For example, the carbon materials used as the conductive additive are often carbon blacks such as acetylene black. However, carbon black has a low bulk density and so the density of a carbon black-containing active material layer decreases, leading to a decline in the volumetric capacity density of the battery. Also, because pores exist in the carbon black, binder is taken up therein, sometimes lowering adhesion at the current collector/active material layer interface. In addition, because carbon black is small in size compared with the active material, it tends to be shed by the electrode, which sometimes leads to internal shorting of the battery.
Hence, various problems sometimes arise in secondary batteries which use conductive additive-containing electrodes. However, in cases where, to avoid such a state, no conductive additive is added to the active material layer, the density of the active material layer increases, but the active material layer has a low conductivity and so sufficient charging and discharging do not take place.
Methods that involve inserting a conductive undercoat layer between the current collector and the active material layer have been developed as a way of lowering the battery resistance by increasing adhesion between the current collector and the active material layer and decreasing the contact resistance. For example, Patent Document 1 discloses the art of using, as an undercoat layer, a conductive layer that contains carbon as a conductive filler, and placing this undercoat layer between a current-collecting substrate and an active material layer. It has been demonstrated that, by using a composite current collector having such an undercoat layer, the contact resistance between the current-collecting substrate and the active material layer can be reduced, in addition to which a decrease in capacity during rapid discharge can be suppressed and, moreover, deterioration of the battery can also be suppressed. Similar art is disclosed in Patent Documents 2 and 3 as well. Furthermore, undercoat layers containing carbon nanotubes as a conductive filler are disclosed in Patent Documents 4 and 5.
Using the art disclosed in these patent publications, a lowering of the battery resistance can be achieved, but because the active material layer mentioned in this literature includes a conductive additive, there is a possibility that the above-described problems will arise.